Vapour phase oxidation



United States Patent 3,304,265 VAPOUR PHASE OXIDATION Arthur WallaceEvans, Nunthorpe, and Kenneth Arkless,

Eaglesclilte, England, assignors to British Titan Products CompanyLimited, Billingham, England, a corporation of Great Britain No Drawing.Continuation of application Ser. No. 274,939, Apr. 23, 1963, now PatentNo. 3,228,887, dated Jan. 11, 1966. This application Mar. 10, 1965, Ser.No. 438,762

5 Claims. (Cl. 252-3011) This application is a continuation of copendingUS. application Serial Number 274,939, filed April 23, 1963, now PatentNo. 3,228,887.

The present invention relates to an improved process for the productionof titanium dioxide by the vapor phase oxidation of a titaniumtetrahalide, and to the pigmentary product.

The oxidation of titanium tetrahalides, particularly titaniumtetrachloride, by an oxidizing gas has been previously described, forexample in British patent specification No. 761,770.

For certain purposes, it may be desired to produce titanium dioxidepanicles of limited median weight crystal size, for example below thatnormal for pigments. Such particles can be used for nuclei in a furtheroxidation reaction.

Our copending US. patent application Serial No. 254,- 007, nowabandoned, describes a particular use for such particles of limited sizewherein one starts with particles of a limited size (cg, 0.01 to 0.25micron) and grows them to a larger size.

(The median weight crystal size is that particle size wherein 50% byweight of the particles have a diameter beneath this value and 50% byweight of the particles have a diameter above thi value.)

It is an object of the present invention to provide a process for theoxidation of a titanium tetrahalide in the vapor phase wherein thetitanium dioxide particles produced are of limited size.

Accordingly, the present invention comprises oxidizing a titaniumtetrahalide to titanium dioxide in the vapor phase, in the presence of asource of thorium. It has been found that the presence of a source ofthorium produces smaller product particles.

The oxidation may be carried out by introducing the titanium tetrahalideand the oxidizing ga into an empty reactor through a burner, wherein thetemperature of the reaction is maintained by supplying heat from anexternal source such as pre-heating the reactants before mixing, or byburning a fuel within the reaction chamber.

Alternatively, the oxidation may be carried out in a reactor wherein thereaction zone has a fluidized bed of inert particles. A considerablepart of the heat of reaction is retained in these particles and servesto heat the incoming reactants so that the reaction can if desired bemaintained autothermally.

The titanium tetrahalide is preferably titanium tetrachloride, and it ispreferably introduced into the reactor as a vapor. The tetrafluoride isunsuitable for use in the present invention and the term tetrahalide" asused herein does not include the tetrafluoride.

The oxidation is preferably carried out by mixing the tetrahalide withoxygen at a suitable temperature. The oxygen may be in the form of amixture with an inert gas, if desired, for example air oroxygen-enriched air.

The source of thorium is most conveniently a salt of the element,particularly a salt which is water soluble and/or which possessesappreciable vapor pressure at a moderate temperature, for example at atemperature below about 1,000 C. and preferably below about 800 C.

"ice

Water-soluble compounds of thorium are particularly suitable for use inthe process when the latter is carried out in a fluidized bed, whereascompounds of thorium which can be converted to a vapor are particularlysuitable for use in a process wherein the reactants are introduced intoan empty reactor through a burner.

Thorium tetrachloride has been found to be particularly suitable forboth processes since it is water-soluble and it sublirnes at atemperature within the range of about 720 to 750 C. Examples of othercompounds which may be used are the sulfates, phosphates, carbonates,nitrates and oxalates of thorium. Also envisaged for use in the processof the present invention are thorium oxide, thorium hydroxide, thoriumsulfide and thorium platinocyanide.

Where the process is carried out in a fluidized bed, the source ofthorium may be introduced as a solid or in solution, for example in anaqueous solution. One method is to slurry the particulate bcd materialwith a solution of the source of thorium and to dry the resulting slurrybefore using the material in the bed. Alternatively, the source ofthorium may be in a form suitable for use as a proportion of theparticulate material of the bed, for example the bed may contain aproportion of thorium dioxide of a suitable particle size.

Where the process is carried out with the continuous or intermittentremoval of bed particles enlarged by accretion and their replacement bysmaller particles, the latter can be mixed with the required amount of asolid source of thorium before introduction into the bed.

If the thorium source is added to the bed batchwise, sufficient of thesource may suitably be added to give a concentration of thorium in thebed in the range of 0.01% to 5%, preferably in the range of 0.5% to 2%,based upon the weight of the fluidized bed. When continuous additions ofthe source are made to the bed, or when the process is carried out byintroducing the reactants into the reaction chamber through a burner, iti preferred to base the amount of thorium added upon the amount oftitanium dioxide produced, and under these circumstances suflicient ofthe compound containing the thorium may be added to give a theoreticalconcentration of thorium in the titanium dioxide produced of from 0.001%to 10%, preferably from 0.01% to 1.0%, by Weight of such titaniumdioxide.

Where the process is carried out in a fluidized bed, the bed is normallymaintained in the fluidized state by passing the reactants into the bedat suitable flow rates. If desired, however, other gases can beintroduced into the bed to assist fluidization. When the oxidizing gasis air, the nitrogen in the air may act as such other gas. Theintroduction of such gas may, however, cause difliculty in the recoveryof the halogen, for example chlorine, liberated during the oxidation.

The fluidized bed should normally be maintained at a temperature in therange of 800 C. to 1400 C., preferably 900 C. to 1100 C., during thepresent process.

Where the fluidized bed process is carried out on a large scale, forexample in a well-lagged reactor having an internal diameter greaterthan about 18", the heat losses may be so small that the heat ofreaction from the oxidation of the titanium tetrahalide will maintainthe desired bed temperature and the process will be autothermal. Whenthe heat losses are too great, for example when the process is carriedout on a smaller scale, it may be necessary to supply heat from anoutside source. This may be provided by electrical heating, bypreheating one or more of the gases introduced into the fluidized bed,or by burning a fuel, for example carbon monoxide, in the bed. In thelatter case sutficient oxidizing gas must be introduced to burn the fuelin addition to oxidizing the titanium tetrahalide to titanium dioxide.

The particulate material forming the fluidized bed may be any materialof suitable particle size which does not adversely affect the oxidationof the titanium tetrahalide. A preferred particle size is one in therange 50 to 2,000 microns, particularly 100 to 1,000 microns. Materialssuch as silica, zircon, alumina, zirconia, titanium dioxide or mixturesthereof may be used, or alternatively a material or a proportion ofmaterial may be used in the bed which contains a source of thorium asrequired by the present invention, for example material containingthorium dioxide, as previously mentioned.

In addition, to the titanium tetrahalide, the oxidizing gas and thesource of thorium, minor proportions of other gases or vapors may heintroduced into the reactor during the reaction. Such other substancesmay include rutilizing agents, such as aluminum halides, zirconiumtetrahalides and water vapor, and other titanium dioxidecrystal-modifying agents such as silicon halides (for example silicontetrachloride), antimony halides (for example antimony pentachloride)and phosphorus halides (for example phosphorous trichloride). Inparticular, it may be advantageous to introduce a small quantity of analuminium halide and a silicon halide, if the process is carried out ina fluidized bed, in order to provide a softer accretion of titaniumdioxide on the bed particles as described in our co-pending US. patentapplication Serial No. 126,310, now Patent No. 3,219,468.

The titanium dioxide produced and entrained in the effluent gases may berecovered, if desired, in a known manner. However, the effluent gasescarrying the titanium dioxide particles (which are generally smallerthan those produced in the absence of thorium) are particularly suitablefor use in the process described in our copending US. patent applicationSerial No. 254,007, now abandoned. For this purpose the etlluent gasescontaining the titanium dioxide particles may be passed directly througha second reaction zone into which at least two additional introductionsof titanium tetrahalide and/or oxidizing gas (and preferably more) aremade as the gas stream and the entrained titanuim dioxide particles passthrough it.

It is believed that under these latter circumstances the existingtitanium dioxide particles act as nuclei upon Which is deposited thetitanium dioxide formed from the oxidation of the titanium tetrahalidein the second reaction zone. The titanium dioxide particles introducedinto the second reaction zone thus grow in size to the optimum pigmentparticle size and also appear to be converted to pigment particleshaving a smaller size range than has been obtainable hitherto (i.e. thefinal product has a more uniform particle size).

It is preferred that the temperature of the efiluent gases and titaniumdioxide particles produced by the process of the present invention bemaintained at such a level when they are transferred to the secondreaction zone that the tetrahalide and oxidizing gas introduced into thesecond reaction zone react rapidly without the necessity of supplyingadditional heat to this zone. For this purpose the efiluent gases andtitanium dioxide particles from the present process should be at atemperature of at least 600 C., and preferably at least 800 C., whenintroduced into the second reaction zone.

Where the vapor phase oxidation of a titanium tetrahalide is carried outaccording to the present process in a fluidized bed it has been foundthat the accretion of titanium dioxide on the bed particles is generallyreduced by the presence of thorium.

A further advantage of the present invention is that it makes possiblethe production of a radioactive pigment, the common forms of thoriumbeing radioactive.

Example I A 3" silica tube was set up having a silica plate fused acrossthe lower end through which protruded two tubes for the introduction ofreactants. The silica tube was surrounded by an electric furnace.Titanium dioxide particles of a size range 44+72 B.S.S. were slurriedwith an aqueous solution of thorium tetrachloride containing sutficientof the latter compound to give a concentration of 0.5% by weight, ofthorium tetrachloride on the titanium dioxide particles. The slurry wasthen dried while being stirred and sufficient of the treated particleswere placed in the silica tube to give a static bed height of 6".

The electric furnace was then switched on and the bed was fluidized withnitrogen. When the temperature of the bed reached 1050 C. the How ofnitrogen was stopped and oxygen at a rate of 18 litres per minute (atN.T.P.) was introduced through one tube and titanium tetrachloridevapour was introduced through the other tube at a rate equivalent to 55ml. of liquid titanium tetrachloride per minute. The oxygen streamcontained sufficient aluminium trichloride vapour to give 3% of alumi'na, based upon the Weight of titanium dioxide produced, and the titaniumtetrachloride stream contained sufficient silcon tetrachloride vapor togive 0.25% of silica, based upon the weight of titanium dioxideproduced.

The experiment was continued for 30 minutes. The titanium dioxideproduced was collected. The median weight crystal size of the productwas estimated from a photograph obtained by means of an electronmicroscope and the Standard Deviation was also estimated.

The Standard Deviation of the crystal size is calculated from the curveobtained when the particle size of the product (in microns) is plottedagainst the weight percentage of the product which is less than a givenparticle size. The former value is expressed on a logarithmic scale andthe latter value on a probability scale. The Standard Deviation is takenas the ratio between the particle size at 84% and that at 50%.

Example 2 A similar experiment to that described in Example 1 wascarried out wherein the bed material was slurried with sufficient of theaqueous solution of thorium tetrachloride to produce a concentration of1% of thorium tetrachloride, by weight, on the bed particles.

Example 3 This was a control experiment not using the process of theinvention. It was similar to that described in Examples 1 and 2 exceptin that the bed material contained no thorium tetrachloride.

The results obtained in Examples 1, 2 and 3 are given in the tablebelow.

The process described in Example 1 was repeated but in addition furtherquantities of titanium tetrachloride and oxygen were passed into thereactor by means of two injectors which were introduced through the topof the reactor and which projected into the space above the fluidizedbed. Each injector consisted of a silica pipe of 6 mm. internal diametersealed at the bottom and having two 4 mm. diameter holes in the side sopositioned that the holes were 15 inches and 24 inches respectivelyabove the base of the silica tube. One injector was connected to asource of titanium tetrachloride vapour and the other to a source ofoxygen.

Titanium tetrachloride vapor, preheated to 200 C., was passed into oneinjector at a rate equivalent to 110 ml. per minute of liquid titaniumtetrachloride and oxygen was passed into the other injector at a rate of36 litres per minute (estimated at N.T.P.). The total flow of titaniumtetrachloride into the reactor, therefore, was 165 ml. per minute (asliquid titanium tetrachloride) and the total flow of oxygen was 54litres per minute.

The titanium dioxide entrained in the effluent gases had excellentpigmentary properties with a tinting strength of 1720 (on the ReynoldsScale); a rutile content of 99.2% and a mean crystal size of 0.22micron. The Standard Deviation was 1.35.

The amount of titanium dioxide retained on the bed particles was 12.3%of the total weight of titanium dioxide theoretically derived from thetitanium tetrachloride supplied to the reactor.

Example 5 The beta-activity of the product of the invention was measuredwith a Geiger counter by taking a two gram sample of the product whichwas approximately 14 months old, compressing the material into a disc 16mms. in diameter and placing the disc thus formed close to the endwindow of the Geiger counter of similar diameter. The activity was 43counts per minute. Allowance was made in this experiment for counts dueto background radiation.

What is claimed:

1. Particles for use in a fluidized bed of a TiO vapour phase oxidationprocess comprising a member selected from the group consisting ofsilica, zircon, alumina, zirconia, titanium oxide, and mixtures thereof,said member containing a theoretical concentration of 0.01 to 5 percentby weight thorium.

2. A method of producing fluid bed particles containing thorium for usein a fluidized bed process for the production of titanium dioxide by thevapor phase oxidation of a titanium tetrahalide selected from the groupconsisting of TiCl TiBr and Til which comprises slurrying particulatematerial selected from the group consisting of silica, zirconia,alumina, zircon, titanium dioxide, and mixtures thereof with a solutioncontaining a source of thorium, and drying the resulting slurry suchthat the dry particulate material contains 0.01 to 5 percent by weightof the thorium source.

3. A method of preparing fluid bed particles for use in a fluidized bedprocess in the production of pigmentary titanium dioxide by the vaporphase oxidation of a titanium tetrahalide selected from TiCl TiBr andTiI which comprises slurrying particulate material capable of beingfluidized at a temperature of at least 800 C. with a solution of athorium source, drying the resulting slurry, and recovering dryparticulate material containing 0.01 to 5.0 percent by weight of thethorium source.

4. The method of claim 3 wherein the particulate material is selectedfrom silica, zirconia, alumina, zircon, titanium dioxide, and mixturesthereof.

5. The method of claim 3 wherein the particulate material has a particlesize in the range of to 2,000 microns.

References Cited by the Examiner UNITED STATES PATENTS 2,347,496 4/1944Muskat et al l06299 X CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGE'IT, Examiner.

A. G. BOWEN, S. TRAUB, Assistant Examiners.

1. PARTICLES FOR USE IN A FLUIDIZED BED OF A TIO2 VAPOUR PHASE OXIDATIONPROCESS COMPRISING A MEMBER SELECTED FROM THE GROUP CONSISTING OFSILICA, ZIRCON, ALUMINA, ZIRCONIA, TITANIUM OXIDE, AND MIXTURES THEREOF,SAID MEMBER CONTAINING A THEORETICAL CONCENTRATION OF 0.01 TO 5 PERCENTBY WEIGHT THORIUM.
 2. A METHOD OF PRODUCING FLUID BED PARTICLESCONTAINING THORIUM FOR USE IN A FLUIDIZED BED PROCESS FOR THE PRODUCTIONOF TITANIUM DIOXIDE BY THE VAPOR PHASE OXIDATION OF A TITANIUMTETRAHALIDE SELECTED FROM THE GROUP CONSISTING OF TICL4, TIBR4 AND TIL4WHICH COMPRISES SLURRYING PARTICULATE MATERIAL SELECTED FROM THE GROUPCONSISTING OF SILICA, ZIRCONIA, ALUMINA, ZIRCON, TITANIUM DIOXIDE, ANDMIXTURES THEREOF WITH A SOLUTION CONTAINING A SOURCE OF THORIUM, ANDDRYING THE RESULTING SLURRY SUCH THAT THE DRY PARTICULATE MATERIALCONTAINS 0.01 TO 5 PERCENT BY WEIGHT OF THE THORIUM SOURCE.